fix(nip46): restored sessions no longer demand a connect secret re-echo

Live Amber sign-in (Sep 25) paired fine, but every restart died with
'the signer did not echo the connection secret': the restore re-sends
connect with the ORIGINAL pairing secret, and an already-approved
signer legitimately answers 'true' without re-echoing — NIP-46 reserves
the echo for proving possession during the INITIAL pairing, and Amber
proved it once. Requiring it on re-dial made session restore fail
100% against real Amber (the e2e missed it because its fake answered a
plain ack with no secret in play).

ConnectUri gains a 'restore' flag (set only by reactivate_saved_sessions).
Fresh handshakes still fail closed on a wrong echo. The restore path's
anti-spoofing is expected_identity in adopt_identity — a peer answering
as any other account is refused, and only the real key holder can
decrypt traffic on the stored conversation key. Log now says
'secret validation: SKIPPED (restored session)' and proceeds.

e2e: run_fake_amber now mirrors Amber's ack shapes (plain ack on first
pairing; 'true' when the client re-presents a secret) and the restore
test seeds a pairing secret so it exercises exactly the live failure —
it fails without the fix and passes with it.

cargo test 216 unit + 5 e2e green; clippy --all-targets 0 warnings;
fmt clean; release rebuilt.

src/signer/nip46_client.rs

@@ -112,6 +112,15 @@ struct ConnectUri {
     relays: Vec<RelayUrl>,
     secret: Option<String>,
     permissions: Option<Nip46Permissions>,
+    /// True when this session was rebuilt from a saved vault row
+    /// (`reactivate_saved_sessions`) rather than dialed from a fresh URI.
+    /// A restored session re-sends `connect` with the ORIGINAL pairing
+    /// secret, but an already-approved signer answers `true` without
+    /// re-echoing it (NIP-46 reserves the echo for proving possession
+    /// during the initial pairing) — so the echo check is skipped and the
+    /// anti-spoofing is `expected_identity` in `adopt_identity` instead:
+    /// answering as any other account fails closed.
+    restore: bool,
 }
 
 /// The NIP-46 client signer.
@@ -274,6 +283,7 @@ impl Nip46ClientSigner {
             relays,
             secret,
             permissions,
+            restore: false,
         })
     }
 
@@ -630,6 +640,11 @@ impl Nip46ClientSigner {
             relays,
             secret: connect_secret,
             permissions: None,
+            // Restored vault session: already-approved signers answer the
+            // re-`connect` with `true` instead of re-echoing the pairing
+            // secret; the handshake skips the echo and binds identity via
+            // `expected_identity` instead.
+            restore: true,
         };
         let signer = self.clone();
         let task = tokio::spawn(async move {
@@ -1082,6 +1097,7 @@ impl Nip46ClientSigner {
             relays,
             secret: Some(secret),
             permissions: connection.permissions.clone(),
+            restore: false,
         };
         let paired = {
             let mut inner = self.inner.lock().await;
@@ -1565,8 +1581,13 @@ impl Nip46ClientSigner {
             let handshake = self.clone();
             let peer = uri.peer;
             let expected_secret = uri.secret.clone();
+            let restore = uri.restore;
             tokio::spawn(async move {
-                if let Err(e) = handshake.clone().run_handshake(peer, expected_secret).await {
+                if let Err(e) = handshake
+                    .clone()
+                    .run_handshake(peer, expected_secret, restore)
+                    .await
+                {
                     handshake.fail(e);
                 }
             });
@@ -1998,6 +2019,7 @@ impl Nip46ClientSigner {
         self,
         peer: PublicKey,
         expected_secret: Option<String>,
+        restore: bool,
     ) -> Result<(), String> {
         // Resolve the connect secret ON-DEMAND from the vault — the single
         // source of truth. Fail-closed: if the vault cannot produce the
@@ -2053,16 +2075,31 @@ impl Nip46ClientSigner {
         // Anti-spoofing: per NIP-46, a signer answering a nostrconnect://
         // (secret-carrying) handshake must echo the secret back. A mismatch
         // means something other than the intended signer answered.
+        //
+        // RESTORED sessions are exempt from the echo: the secret they resend
+        // is the ORIGINAL pairing secret, and a signer that already approved
+        // this client answers `true` instead of re-echoing (an echo would
+        // re-prove possession that the initial pairing proved once). Real
+        // Amber does exactly this, so requiring the echo made every restart
+        // fail with "did not echo the connection secret". The restore path's
+        // anti-spoofing is `expected_identity`, enforced in adopt_identity:
+        // anything that answers as another account is refused, and only the
+        // key holder can decrypt our traffic on the stored conversation key.
         if let Some(expected) = &expected_secret {
             let trimmed = ack.trim();
-            if trimmed != expected.as_str() {
+            if trimmed == expected.as_str() {
+                eprintln!("[NIP46] secret validation: PASS (connect ack echo matched)");
+            } else if restore {
+                eprintln!(
+                    "[NIP46] secret validation: SKIPPED (restored session; ack={trimmed}) — identity will be re-proved"
+                );
+            } else {
                 eprintln!("[NIP46] secret validation: FAIL (connect ack echo did not match)");
                 return Err(
                     "The signer did not echo the connection secret; refusing the connection."
                         .to_string(),
                 );
             }
-            eprintln!("[NIP46] secret validation: PASS (connect ack echo matched)");
         }
 
         self.adopt_identity(peer).await

tests/nip46_e2e.rs

@@ -284,10 +284,19 @@ async fn run_fake_amber(relay_url: String, comms: Keys, identity: Keys, approval
 
         let response: Value = match method {
             "connect" => {
-                // Simulate a human tapping "approve" in Amber.
+                // Simulate a human tapping "approve" in Amber. Amber's
+                // ack SHAPE depends on the connection state: a first-time
+                // pairing (no secret in params) gets a plain ack; an
+                // ALREADY-APPROVED connection re-dialing with the stored
+                // secret gets `true` WITHOUT a secret echo (live Amber,
+                // Sep 25 — the client must not demand an echo there).
                 tokio::time::sleep(approval_delay).await;
+                if req["params"].as_array().is_some_and(|p| p.len() > 1) {
+                    json!({"id": id, "result": true})
+                } else {
                     json!({"id": id, "result": "ack"})
                 }
+            }
             "get_public_key" => json!({"id": id, "result": identity.public_key().to_hex()}),
             "sign_event" => {
                 let unsigned_json = req["params"].get(0).and_then(|v| v.as_str());
@@ -1189,6 +1198,18 @@ async fn nip46_session_restore_redials_and_refuses_wrong_identity() {
     // (The old instance's listener task is left running on purpose — the
     // live process exiting is modeled by the new instance, not by
     // `disconnect()`, which revokes and wipes the stored key.)
+    //
+    // Seed a pairing secret at the identity ref first: a QR pairing stores
+    // one, and the restore re-sends it — real Amber then answers `true`
+    // WITHOUT echoing (already-approved connection), which the fake models.
+    // Without the restore-mode skip this handshake fails "did not echo the
+    // connection secret" (the exact live Sep 25 failure).
+    {
+        let mut g = app.lock().await;
+        keynectr::vault::store_connection_secret(&mut g.vault, None, &ref_id, "restore-secret-123")
+            .unwrap();
+        g.save_vault().unwrap();
+    }
     let signer2 = Nip46ClientSigner::new(app.clone());
     let restored = signer2
         .reactivate_saved_sessions()